Password access problem (XP)

This is a case of the blind relaying for the blind, but here goes...

My neighbour has two PCs (call them A & [img]/forums/images/smilies/cool.gif[/img] networked, both running XP Pro. A can access the files on B, but B can't access the files on A, even though B can "see" A as part of the network. There is a request for a password to enable such access, but he has no idea of what that might be (A was apparently "set up" by someone else). The only good news is that he can fully access everything on each of them independently.

I did run across a thread about a registry setting, which he changed from 1 to 0 on A, as suggested, but this made no difference. From the sound of things, he's stuck at the password screen. Are there any obvious things to try, or possible workarounds here?